An ultrasonic transducer is used for, for example, a diagnostic apparatus for a tumor inside a human body or others. Conventionally, an ultrasonic transducer mainly using vibration of a piezoelectric body is used. However, as a MEMS (Micro Electro Mechanical System) technique has been developed in recent years, a capacitive micromachined ultrasonic transducer (CMUT) has been currently developed, in which a vibrating portion having a structure that a hollow portion is sandwiched between upper- and lower-layer electrodes is formed on a silicon substrate.
The CMUT has advantages such as a wider usable ultrasonic frequency band than that of the transducer using the piezoelectric body or high resolution. Also, since the CMUT is manufactured by using LSI (Large Scale Integration) process techniques, the CMUT can be microfabricated. More particularly, it is considered that the CMUT is essential for arranging one ultrasonic element in array and independently controlling each ultrasonic element. The reason is that, while the number of wires in a cell array is very large because a wire is required for each ultrasonic element, the wires in the CMUT are simply formed because the CMUT is manufactured by using the LSI process technique. Also, the reason is that, in the CMUT, a processing circuit for signals from an ultrasonic transmitting/receiving unit can be embedded on one semiconductor chip.
The technique for the ultrasonic transducer is disclosed in, for example, U.S. Pat. No. 6,271,620B1 (Patent Document 1).